The present invention concerns processing of integrated circuits and pertains particularly to in-situ monitoring of wafer thickness during backgrinding.
Before dividing silicon wafers into dies and packaging the resulting integrated circuits, it is often necessary to grind the back of a wafer until the wafer is a predetermined thickness. Such wafer backgrind processes control grind thickness using a feedback loop where wafer thickness is measured at the center of the product wafer. The center wafer measurement is used for feedback. Test wafers or product wafers are used to monitor thickness and thickness range of the grind process. Test wafers are used to initially verify the thickness range of the grind process after maintenance, a change in grind thickness or other change in the grind process. The feedback uses the thickness of an already ground product wafer to provide feedback during grinding a new product wafer. However, there are several deficiencies in this system.
For example, it is impossible to monitor uniformity on product wafers without taking ex-situ (i.e., not during actual performance of grinding) measurements on a previously ground batch of product wafers. Generally, such monitoring requires a separate metrology tool and thickness measurement step. Also, a separate measurement stage on the grinder itself is required for checking the thickness of the wafer. Using this methodology, a certain lack of uniformity in wafer thickness can result. The lack of uniformity in wafer thickness can result in wafer bow and stress. Further, lack of grind uniformity can result in stresses in silicon and thin films placed on the wafer surface. These stresses can lead to cracks and voids in the silicon substrate. Also hillocks and film peeling can occur on the device side of the wafer if stresses in the wafer are not kept in check. Further, sometimes wafers break when grinder setup is not done properly. Additionally, the above-described methodology introduces a significant time lag between the actual grinding and the detecting of a lack of uniformity or improper thickness in a batch of wafers.